Centrifugal clutches of this type are used in portable apparatus and especially in apparatus driven by an internal combustion engine, such as motor-driven chain saws, in order to keep the tool such as a saw chain from being driven when the motor is idling. This makes it possible to put the apparatus down safely during work pauses without having to turn off the motor. When the work is resumed, the motor is accelerated, which engages the centrifugal clutch and the tool and the drive unit are reconnected to one another.
U.S. Pat. No. 4,625,849 shows a coupling of this type. The clutch described in this patent comprises a carrier which is provided for attachment to the drive shaft and on which carrier three jaw-like centrifugal weights are arranged in such a manner that they can be radially displaced. A tension spring is located approximately tangentially between each two centrifugal weights. The springs bias the centrifugal weights into their rest positions in the vicinity of the rotational axis of the carrier. The centrifugal weights are surrounded at a small spacing by a drum which forms the output part of the clutch and which is provided for a drive connection with the part for driving a tool, for example, the drive sprocket of a chain saw. Centrifugal weights and tension springs are designed in such a manner that the centrifugal weights are biased by the pull force of the springs in their position in the vicinity of the rotational axis when the motor is idling. As the speed of the motor increases, the centrifugal weights move outwardly because of centrifugal force counter to the spring force until they frictionally engage the surrounding clutch drum so that the drive connection between motor and tool is established. The centrifugal weights and clutch drum disengage in a corresponding manner as the motor speed decreases.
A disadvantage, especially in the case of motor-driven chain saws equipped with the described clutch, is the fact that once the saw chain has become stuck in the wood being cut, it can no longer be freed by means of its own drive since as a rule the torque available when the centrifugal clutch is engaged is not sufficient and the clutch slips. The chain therefore usually has to be freed by removing the restraint mechanically such as by inserting a wedge. This takes time and has an adverse effect on the work to be performed. Also, the apparatus is exposed to unnecessary mechanical stresses which increase the wear of the tool driven via the clutch and drive unit.